1. Field of the Invention
The present invention relates in general methods and devices for producing or reforming a bellows, and more particularly methods and devices for producing or reforming a metal bellows installed in a flexible tube that is disposed in an exhaust pipe line of an automotive internal combustion engine to absorb undesired vibration of the exhaust pipe line and compensate relative displacement between two portions of the exhaust pipe line.
2. Description of the Prior Art
In order to clarify the task of the present invention, one conventional flexible tube to which a bellows is practically applied will be briefly described with reference to FIGS. 5 and 6 of the accompanying drawings. The conventional flexible tube depicted by FIGS. 5 and 6 is described in detail in Japanese Utility Model First Provisional Publication 61-187916.
In FIG. 6, there is shown the flexible tube which is generally designated by numeral 100. As is seen from this drawing, the flexible tube 100 is a device arranged to connect upstream and downstream exhaust pipes "UP" and "DP" of an exhaust pipe line in such a manner as to absorb the vibration of the pipe line, and/or compensate a relative displacement between the two pipes "UP" and "DP".
The flexible tube 100 comprises a metal bellows 102 that has an upstream end tightly disposed on a downstream end of the upstream exhaust pipe "UP" and a downstream end tightly disposed on an upstream end of the downstream exhaust pipe "DP". A cover 104 of braided metal wire covers or encloses the bellows 102 having an upstream end tightly disposed on the upstream end of the bellows 102 and a downstream end tightly disposed on the downstream end of the bellows 102. For the tight mounting of the upstream and downstream ends of both the bellows 102 and the cover 104 onto the upstream and downstream exhaust pipes "UP" and "DP", respective metal collars 106 and 108 are tightly disposed on the upstream and downstream ends of the cover 104, as shown. The bellows 102 can absorb vibration transmitted thereto from an internal combustion engine (not shown) through the upstream exhaust pipe "UP". That is, upon receiving vibration, the bellows 102 is subjected to a certain resilient deformation due to the nature thereof, which absorbs the vibration and compensates a relative displacement between the upstream and downstream pipes "UP" and "DP".
The cover 104 functions to restrict an excessive elongation of the bellows 102 and to protect the bellows 102 from being hit by small stones or the like flying from the road. That is, by a certain length, the cover 104, which is constructed of braided metal wire, can axially expand following the elongation of the bellows 102. Thus, when the elongation of the bellows 102 reaches to the certain length, the cover 104 now functions to stop the further elongation of the bellows 102. That is, due to provision of the cover 104, the bellows 102 can be protected from making an excessive elongation. In other words, the bellows 102 can expand axially by a certain length within the cover 104.
For assembling the flexible tube 100, the metal bellows 102 is reformed before being put into the cover 104. That is, the metal bellows 102 is subjected to a so-called "single compression process" for achieving both a dimensional stability of the treated bellows 102 and an appropriate axial flexibility of the treated bellows 102. That is, as is seen from FIG. 5, in this compression process, a blank 102X of the bellows 102 is compressed once to the length "L-a" which is shorter than the normal length "Ls" of the bellows 102. This compression process is positively carried out for the reason originating from an inevitably occurring "spring-back phenomenon" of the compressed bellows 102Y. In fact, due to this spring-back phenomenon, after the compression, the over-compressed bellows 102Y gradually expands to have the normal length of "Ls". Furthermore, due to this compression, the pitch of bulges of the treated bellows 102 becomes small causing each bulge to have a generally .OMEGA.-shaped cross section, which brings about an appropriate axial flexibility or resilient deformation of the bellows 102.
However, it has been revealed that the above-mentioned single compression of the bellows 102X leaves in the bellows 102 a stress (or residual stress) of a type that causes the bellows 102 to expand in an axial direction when heated.
Accordingly, when the flexible tube 100 having the above-mentioned bellows 102 installed therein is practically used, that is, used in an exhaust pipe line of the engine, the entire length "Ls" of the bellows 102 tends to increase due to releasement of the residual stress by the heat of the exhaust gas from the engine. The increase in the entire length "Ls" of the bellows 102 however means a reduction in the certain length by which the bellows 102 can expand axially within the cover 104. That is, a so-called "elongation flexibility" of the bellows 102 is reduced or lowered at the time when the flexible tube 100 is being practically used.
The expansion/contraction of the bellows 102 is carried out while being interrupted by the cover 104. The cover 104 has such a structure as to reduce its diameter when axially expanded. Thus, elongation of the bellows 102 caused by application of the exhaust gas heat thereto brings about elongation of the cover 104 and thus reduces the diameter of the same. Reduction in diameter of the cover 104 narrows an annual space defined between the bellows 102 and the cover 104, which causes an obstacle to the elongation flexibility of the bellows 102. This fact will be understood from the graph of FIG. 4.
The graph of FIG. 4 shows a relationship between the force "F" needed for elongation of a bellows and the elongation "E" of the bellows. In the graph, the solid line curve represents the elongation flexibility possessed by a normally dimensioned bellows 102A installed in the cover 104, which has the normal length "Ls" and the critical elongation "S". As is seen from this graphs in the normally dimensioned bellows 102A, within the region of the critical elongation "S", the elongation "E" of the bellows 102A increases substantially in proportional to the elongation force "F" applied to the bellows 102A. While, when the elongation "E" extends beyond the critical elongation "S", the elongation force "F" suddenly increases and thus the elongation flexibility of the bellows 102A is lowered. The broken line curve represents the elongation flexibility possessed by a bellows 102B somewhat expanded due to the exhaust gas heat applied thereto, which has the length "Ls1" greater than "Ls" and the critical elongation "S1" smaller than "S". As is seen from the graph, in this bellows 102B, due to the reduction in critical elongation, the sudden increase of the elongation force "F" appears at an initial stage of the elongation "E". This means that the elongation flexibility of the bellows 102B is poor as compared with that of the bellows 102A. The phantom line curve represents the elongation flexibility possessed by a bellows 102C somewhat shorter than the bellows 102A due to excessive compression applied thereto, which has the length "LS2" smaller than "Ls" and the critical elongation "S2" greater than "S". Although this bellows 102C can provide a sufficient elongation under the practical use, the shorter initial length "Ls2" of the bellows 102C brings about a difficulty with which the bellows 102C is installed in the cover 104. That is, in this case, the flexible tube 100 assembled fails to have a normally dimensioned structure. According to tests executed by the inventors, the following acts have been further revealed. That is, when a bellows of the length of about 300 mm that has been subjected to a single compression process is left in a room temperature, the bellows is expanded or elongated by about 2 mm. While, when the bellows is practically used or heated by the exhaust gas from an engine, the bellows is expanded or elongated by about 6 to 8 mm. This means that even if the bellows is subjected to the single compression process, a certain stress (or residual stress) is left in the bellows, which causes the bellows to expand in an axial direction particularly when heated. The inventors have further revealed that the stress in the bellows can be sufficiently removed when the bellows is annealed at about 600.degree. C. for about 2 minutes. However, in this case, due to adding of the annealing process, the production process becomes complicated and thus the cost of the flexible tube is increased.